When logging tools are run into the boreholes of oil and gas wells they are generally suspended from logging "wirelines" or cables which run from a reel at the surface of the earth to the depth of the formation or formations to be logged. The device by which the logging tool instrument housings or "tool string" are suspended from the wireline is generally termed a cable head.
Prior art cable heads have incorporated therein a "weak link" or "weak point" at which point the cable head is designed to fail at a predetermined tensile stress in the event the tool string becomes stuck in the borehole, thus permitting retrieval of the wireline and running a fishing tool, as known in the art, to attempt to retrieve the logging tool string. Such prior art cable heads may leave a length of cable on top of the tool string or result after separation in a configuration at the top of the string which is difficult to engage with fishing tools. Additionally, the manner in which the logging cables are secured to prior art cable heads, with a wedging action primarily dependent upon the weight of the tool string as it is suspended in the borehole, may result in premature failure at the wedge assembly rather than at the desired weak link at the predetermined stress. Moreover, many prior art cable heads are not susceptible to use in boreholes containing sour gas, or H.sub.2 S, due to their design as well as the vulnerability to H.sub.2 S of components in the cable head assembly.
Finally, when running logging tools in a borehole, it is often necessary to add weight to the tool string to counteract the buoyancy provided by the hollow logging tool instrument housings. The prior approach to adding weight to the string was to incorporate weights or "sinker bars" in the tool string itself. This approach not only required that the sinker bar include a means of transmitting an isolated electrical signal through it, but required that the sinker bar have a means of sealing pressure out of the instrument string.